On 2025/07/02 19:10, Anthonin Bonnefoy wrote:
Hi,
I've been looking at page cache usage as some of our replicas were
under memory pressure (no inactive pages available) which led to WAL
replay lag as the recovery process had to read from disk. One thing
I've noticed was that the last WAL files are in the pagecache even
after having been replayed.
This can be checked with vmtouch:
vmtouch pg_wal/*
Files: 141
Directories: 2
Resident Pages: 290816/290816 1G/1G 100%
And page-types shows a replayed WAL file in the active LRU:
page-types -Cl -f 000000010000001B00000076
page-count MB long-symbolic-flags
4096 16 referenced,uptodate,lru,active
From my understanding, once replayed on a replica, WAL segment files
won't be re-read. So keeping it in the pagecache seems like an
unnecessary strain on the memory (more so that they appear to be in
the active LRU).
WAL files that have already been replayed can still be read again
for WAL archiving (if archive_mode = always) or for replication
(if the standby is acting as a streaming replication sender or
a logical replication publisher). No?
This patch adds a POSIX_FADV_DONTNEED before closing a WAL segment,
immediately releasing cached pages.
Maybe we should do this only on a standby where WAL archiving
isn't working and it isn't acting as a sender or publisher.
Regards,
--
Fujii Masao
NTT DATA Japan Corporation
